1. Field of the Invention
The present invention relates to an antenna device with a slot antenna structure and, more particularly, to a compact antenna device that radiates a circularly polarized wave.
2. Description of the Related Art
As a circularly polarized antenna with a slot antenna structure, in the related art, there has been proposed an antenna device provided with a two-point power feeding scheme in which a pair of radiation slots, which are equal in length and perpendicular to each other, is excited by feeding power to each of the radiation slots and electric waves radiated from the radiation slots differ in phase by 90 degrees. However, there is a problem in that the above-mentioned antenna device with the two-point power feeding scheme needing a 90-degree phase difference circuit or the like requires a complex circuit configuration.
Recently, there has been proposed an antenna device provided with a one-point power feeding scheme in which an L-shaped radiation slot, which is formed by connecting the respective one ends of first and second slot parts that are arranged spatially perpendicularly to each other, is formed on a conductor plate, and power is fed to a predetermined position in the vicinity of an edge of the radiation slot so that electric waves radiated from the two slot parts differ in phase by 90 degrees (for example, see JP-A-2004-194218 (page 6, FIG. 7)).
The above-mentioned conventional antenna device has an advantage in that it requires a simple circuit configuration since it employs the one-point power feeding scheme. However, when the wavelength of the electric wave is λ, the length of each of the first and second slot parts is about λ/2, such that one side of a conductor plate provided with an L-shaped radiation slot should be more than λ/2 in length. Accordingly, when an operating frequency is, for example, 2.4 GHz, the antenna device needs a conductor plate with an L-shaped radiation slot whose one side is more than 6 cm in length, such that it is difficult to reduce the size of the antenna device.